The present invention relates to mixed matrix membranes, including sieve particles, with the ability to separate a desired gaseous component from gaseous mixtures.
Fischer-Tropsch synthesis is used to convert methane to higher molecular weight hydrocarbons. The first step involves converting the methane to synthesis gas, which is a mixture of carbon monoxide and hydrogen. The synthesis gas is contacted with a Fischer-Tropsch catalyst under conditions of increased temperature and pressure. In addition to desired products, the reaction can also produce methane and carbon dioxide. The methane can be recycled through the synthesis gas generator, but over time, each recycle can lead to increased levels of carbon dioxide. The increased levels of carbon dioxide can adversely affect the Fischer-Tropsch synthesis. It would be advantageous to separate and remove the carbon dioxide from the methane.
Polymeric membrane materials have been proposed for use in gas separation. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (See, for example, Koros et al., J. Membrane Sci., 83, 1-80 (1993), the contents of which are hereby incorporated by reference, for background and review).
The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater concentration of one of the components in the feed mixture than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained at a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane""s permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Various materials (fibers, porous supports, etc.) have been incorporated into polymeric membranes to provide mechanical strength to the membranes. Such materials and the resulting composites are described, for example, in U.S. Pat. Nos. 3,457,170; 3,878,104; 3,993,566; 4,032,454; and 4,341,605. Some composite materials show improved liquid separation properties over the polymer material themselves. For example, U.S. Pat. No. 3,862,030 discloses a composite material that includes inorganic fillers within a polymeric matrix that shows enhanced filtering capabilities for microscopic, submicron particles.
Membrane materials with incorporated materials within a polymeric matrix have also been used for gas separations (see, for example, Zimmerman et al., J. Membrane Sci., 137, 145-154 (1997)). Paul and Kemp (J. Polymer Sci., Symposium No. 41, 79-93, (1973)) disclose a composite material including Type 5A zeolites in silicone rubber. This material purportedly caused a time lag in achieving steady-state permeation by xe2x80x9cimmobilizingxe2x80x9d various gases due to adsorption to the zeolites. However, after saturation of adsorption sites, permeation of the gases reached steady state. Later, Jia et al. (J. Membrane Sci., 57, 289 (1991)) showed that incorporating silicalite (a hydrophobic crystalline silica-based zeolite, described in U.S. Pat. No. 4,061,724) into silicone rubber provided improved selectivities for oxygen/nitrogen separation. The oxygen/nitrogen selectivity (PO2/PN2) increased from the original 2.1 for the silicone rubber to 2.7. Later, Kulprathipanja et al. (U.S. Pat. Nos. 4,740,219 and 5,127,925) introduced silicalite into cellulose acetate and showed slightly enhanced oxygen/nitrogen selectivity (from 2.99 to 3.63). Sxc3xcer et al. (J. Membrane Sci., 91, 77 (1994)) and Third International Symposium on Separation Technology, Antwerp, Belgium, Elsevier Science B. V., (1994) incorporated Type 4A zeolite into a glassy polymer, polyethersulfone, and also showed enhanced oxygen/nitrogen selectivity (from 3.7 to 4.4). Additional studies have examined zeolites in other polymer matrices and/or have explored surface modification techniques (silane coupling, etc.) to improve adhesion between the dispersed zeolite and polymer matrix: Gxc3xcr (J. Membrane Sci., 93, 283 (1994)) and Duval et al. (J. Membrane Sci., 80, 189 (1993) and J. Appl. Polymer Sci., 54, 409 (1994)). Modest or no improvement in oxygen/nitrogen separation was observed.
The art cited above demonstrates the incorporation of various zeolites into various rubbery and glassy polymers for oxygen/nitrogen separation. Although several studies show some enhanced oxygen/nitrogen selectivity, these mixed matrix membranes still do not exhibit the anticipated improvement necessary for commercial application. The most significant work to date was performed by the French Petroleum Institute, where significant selectivity enhancement was reported for methane gas mixtures using flat-sheet membranes containing equal weights of the polyetherimide Ultem(copyright) and zeolite 4A (U.S. Pat. No. 4,925,459).
It would be desirable to provide additional devices, materials and processes for separating gaseous components. The present invention provides such devices, materials and processes.
The present invention is directed to carbon molecular sieve particles and methods of making the same, and to mixed matrix membranes (hereinafter xe2x80x9cmixed matrix membranexe2x80x9d or xe2x80x9cmixed matrix filmxe2x80x9d) and methods of making the same. The membrane comprises carbon molecular sieving entities or particles incorporated into a polymeric membrane. Methods of separating gaseous mixtures using the mixed matrix membrane are also within the scope of this invention. The molecular sieving entities increase the effective permeability of a desirable gas component through the polymeric membrane (and/or decrease the effective permeability of the other gas components), and thereby enhance the gas separation (selectivity) of the polymeric membrane material. Hereinafter, xe2x80x9cenhancedxe2x80x9d permeation properties or xe2x80x9cenhancedxe2x80x9d selectivity refers to this phenomenon. In a preferred embodiment, the membranes are useful for separating carbon dioxide from a gaseous mixture that includes carbon dioxide and methane.
The mixed matrix membrane includes a polymer and small, discrete molecular sieving particles encapsulated in the polymer. The mixed matrix membrane is preferably in the form of a dense film, tube or hollow fiber.
The polymer forming the membrane permits passage of desired gaseous components, in one embodiment carbon dioxide and methane, but at different diffusion rates, such that one of the components, for example either carbon dioxide or methane, diffuses at a faster rate through the polymer. The polymer is generally a rigid, glassy polymer (having high glass transition temperatures, i.e., temperatures above about 150xc2x0 C.), although rubbery polymers or flexible glassy polymers can be used. Examples of rigid glassy polymers include polysulfones, polycarbonates, cellulosic polymers, polypyrrolones, and polyimides. The polymer preferably has, but need not have, selectivity for one or more of the components being separated.
The molecular sieving entity (xe2x80x9cmolecular sievexe2x80x9d or xe2x80x9cparticlexe2x80x9d) is derived from the pyrolysis of any suitable polymeric material that results in an amorphous carbonized structure with short-range order. The polymeric precursor material for pyrolysis can be prepared in any convenient form such as sheets, tubes, hollow fibers, or powder. The carbon molecular sieving entity (xe2x80x9cmolecular sievexe2x80x9d or xe2x80x9cparticlexe2x80x9d) is preferably prepared by subjecting a powdered polymer to pyrolysis conditions to form a carbon molecular sieve powder, which is then milled or otherwise processed to a desired size range. Preferred carbon molecular sieve particles are less than 2 microns in diameter, more preferably less than 1 micron in diameter, and most preferably between 0.1 and 0.9 microns in diameter.
The ratio of particles to polymer is typically between about 0.1 and 1.5, and more preferably, about 0.25 to 1.0 by volume. This corresponds to about 10-60%, and 20-50%, particle volume to total volume. A preferred method for preparing polymeric films is by dispersing the molecular sieving entity in a polymer solution, casting a film of the polymer solution, and evaporating the solvent to form a polymeric film. Optionally, but preferably, the particles are xe2x80x9csizedxe2x80x9d or xe2x80x9cprimedxe2x80x9d before forming the polymer solution. To maximize dispersion of the particles in the polymers, the polymer solution can be subjected to homogenization and/or ultrasound.
The resulting mixed matrix films can be stacked or formed into tubes or hollow fibers, or other conventional shapes used for gas separations. In one embodiment, the gas separations are used to purify hydrocarbon gases obtained during down-hole drilling operations, either in the hole or above-ground. Using the membranes described herein, helium, hydrogen, hydrogen sulfide, oxygen and/or nitrogen can be separated from natural gas hydrocarbons. These gases can also be separated from carbon dioxide.
A preferred method for preparing hollow fibers is to dissolve the polymer, mix in the particles, and extrude the polymer/molecular sieving entity blend through a tubular capillary nozzle with a core fluid used for the purpose of retaining the hollow fiber shape.
Any gases that differ in size, for example nitrogen and oxygen or ethylene and ethane, can be separated using the membranes described herein. In one embodiment, a gaseous mixture containing methane and carbon dioxide can be enriched in methane by a gas-phase process through the mixed matrix membrane. In other embodiments, the membranes are used to purify helium, hydrogen, hydrogen sulfide, oxygen and/or nitrogen.
For a carbon molecular sieving (CMS) particle to impart an enhanced selectivity to a mixed matrix membrane, the high selectivity transport path through the particle must go from one side of the particle to the other in the general direction of gas flow through the membrane. Previously it was believed that to obtain this property, the CMS particle had to be prepared from pyrolysis of a thin supported or unsupported film or hollow fiber to ensure selective passage through the carbon. Furthermore, it was not known if milling CMS films or fibers into fine particle sizes would retain the desired molecular sieving property of the former. In the absence of this property, the gas would either not pass through the carbon particle if the pores were dead-ended, or only pass through large mesopores which are not selective, such that a mixed matrix membrane made with this particle would not exhibit enhanced selectivity. Pyrolysis of a polymer powder was not suggested. It has surprisingly been found that if a polymer powder is pyrolyzed according to the present invention, enhanced selectivity is obtained when powder is used to make mixed matrix membranes. In some instances, such as when the polymer powder particles are large (greater than 2 microns), milling these particles after pyrolysis assists in freeing up the transport path through the particle.